The present invention relates generally to incoherently beam combined (IBC) lasers and, more particularly, to a system for increasing the available power and/or bandwidth of an IBC laser.
In many applications utilizing laser sources, it is desirable to use a source providing diffraction limited power within a specific bandwidth. This bandwidth may be comprised of multiple distinct wavelengths or as a spectrally smooth or continuous distribution of wavelengths.
A suitable source for many of these applications is an Incoherently Beam Combined or IBC laser. In an IBC laser, an external cavity is used to incoherently combine the output from each of an array of single transverse mode, laser gain elements. The external cavity is comprised of the back facet of each gain element and a common output coupler. A reflective or transmissive dispersive element, such as a grating, is located within the cavity and is used to simultaneously force each element of the array to oscillate at a distinct wavelength and combine all of the beams. By using a large number of laser gain elements or emitters, an IBC laser can generate a relatively high power, diffraction limited, laser beam.
In order to increase the power and/or bandwidth of an IBC laser, additional laser gain elements or emitters must be added to the array. Although the F-number of the IBC resonator optics is set by the divergence of an individual emitter and thus is unaffected by an increase in array width, the field angle over which the IBC optics must operate is directly driven by the array width and thus the number of emitters. Consequently, as the number of emitters is increased, the resonator cavity optics become increasingly difficult to manufacture in terms of size, optical design complexity, and cost.
Accordingly, what is needed in the art is an optically simple IBC system that can operate over the relatively large field angles required by a wide laser gain array. The present invention provides such a system.
The present invention provides a method and apparatus that enables an IBC system to operate over a large field angle, thus allowing wide laser gain arrays to be used in a compact package. The IBC optical cavity is formed by the combination of a high reflectance coating applied to the back facet of each emitter of the gain array and an external output coupler. Located within the IBC optical cavity is a wavelength dispersive element, such as a diffraction grating, and a collimating optic, the shape of which may be aspheric.
According to the invention, a lens is placed within the IBC optical cavity, the lens being located between the laser gain array and the collimating optic. In at least one embodiment of the invention, the lens is comprised of a single cylindrical lens that reduces the divergence of the light emitted by the individual emitters in the fast axis while having negligible impact on the divergence of the light in the slow axis. Preferably the divergence in the fast axis is reduced to match that of the slow axis, thereby relaxing the on-axis F-number requirements of the cavity optics to those of the slow axis. The lens shape may be aspheric.
In at least a second embodiment of the invention, the lens placed between the laser gain array and the collimating optic is comprised of an array of individual lens elements, each lens element corresponding to an individual emitter of the array. The lens elements reduce the divergence of each emitter in the fast axis, preferably such that the divergence in the fast axis matches the divergence in the slow axis.
In at least a third embodiment of the invention, the lens placed between the laser gain array and the collimating optic corrects for the astigmatism of the edge emitters. This optical correction can be accomplished with a single lens array comprised of a plurality of lens elements, each of which is an aspheric lens having a different focal length in the slow and fast axes. The lens shape may be aspheric in both the slow and fast axes. Alternately, a single cylindrical lens acting substantially on one axis of the emitter array can be combined with an array of cylindrical lenses acting substantially on the orthogonal axis of the emitter array to correct for the astigmatism of the edge emitters.